Many types of techniques are utilized in order to condition and transmit digital data over various analog mediums, such as through the air in the case of Radio Frequency (RF) communications, or over wire in the case of telephone network communications. Typically, most schemes involve the modification of some base, analog “carrier” waveform.
In the case of digital data communications, the raw digital data stream can be used directly in order to cause frequency changes in the carrier frequency itself. Since the digital data has only two possible states, 0 and 1, this represents what is referred to as “binary modulation.” It is appreciated that this type of modulation is frequently used in low speed data communications. The general term for this type of data transmission is Frequency Shift Keying (FSK).
It is understood that FSK Modulation involves shifting the frequency of an analog carrier sine wave above, and below, the frequency of the carrier signal itself. The specific frequency of the carrier, as well as the amount of +/− frequency shift, can be defined as desired, or chosen to be in compliance with existing specifications for FSK communications.
Once a digital data stream has been encoded, and a resulting modulated analog waveform has been composed, the analog signal is then transmitted over a communication medium. At the other end of the communications link is a receiving device whose responsibility is to demodulate the incoming analog signal and re-create the original digital data stream.
Over the years, many schemes have been employed to perform the binary-demodulation function. The techniques currently in use fall into one of two general categories: Analog Domain Processing or Digital Domain Processing.
The following has to do with Analog Domain Processing. Specifically, performing demodulation in the Analog Domain typically involves the use of two analog bandpass filters. These filters have their pass-band carefully selected and controlled in order to allow only one of the two possible frequencies to pass. Another circuit that follows the bandpass filter stages makes a determination as to which of the two filters has the highest output. Based on this determination, the decision circuit presents a binary “1” or “0” to the input of yet another circuit whose function is to re-frame the individual data bits into digital words. This re-framing function operates over a predefined set of timing parameters.
While this technique certainly does work, there are disadvantages associated with it. For example, it does not lend itself to implementation using modern, low cost digital microprocessors. The cost of the analog circuitry typically required to perform the demodulation can far exceed the cost of the digital microprocessor whose function is to ultimately operate on the communicated data.
Furthermore, modern appliances and personal electronic devices continue to be reduced in physical size. The additional electronic components usually necessary to implement the analog functions makes this type of demodulator unsuitable for use in these types of applications.
The following has to do with Digital Domain Processing. Most recently, a new type of microprocessor, known as a Digital Signal Processor (DSP), has begun to replace the functionality of certain analog circuits. A DSP has the ability to perform a large number of mathematical computations very quickly. This is due to the architecture of the device itself. By adding this ability to a digital microprocessor, it becomes possible to perform the filtering, decision, and re-framing functions on a single device.
While DSPs have proven to be valuable in demodulator applications, as well as digital communications in general, there are disadvantages associated with them. For example, DSPs are relatively high cost components. In many communication systems, the high component cost can be tolerated due to the very high level of performance, and subsequent data transfer rates that can be achieved. Comparable data transfer rates using analog circuitry would carry a much higher cost, thereby making the DSP an attractive alternative.
However, there is a point where the higher cost of the DSP, or the cost and complexity of Analog Domain processing, is uneconomical for certain end products. This situation is occurring more frequently due to the fact that many low cost consumer products are now being enabled for digital communications of some type.